Remove the special case hack for getting zero values at the end of a generator.  This will make this:
Remove the special case hack for getting zero values at the end of a
generator.  This will make this:

  (generator (yield 1) (yield))

also repeat #<void> when it's done which will be very confusing.  Better
just add a note in the docs on (generator (yield 1)) returning no values
when it's done.

Also, remove some of the `case-lambda' optimizations...

svn: r17983

mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket